The present application relates to a light-transmitting electric conductor excellent in light-transmitting property and electrically conductive property, a method of manufacturing the same, a destaticizing sheet, and an electronic device. More particularly, the application relates to a light-transmitting electric conductor which uses carbon nanolinear (nanofilamentous) structures such as carbon nanotubes, and a method of manufacturing the same.
Transparent conductive films have been used as transparent electrodes for electronic apparatuses involving entrance/exit of light, such as liquid crystal display s and touch panels, destaticizing films for light-transmitting members on which deposition of dust is undesirable, such as partitions in clean rooms. As a material for such transparent conductive films, there have been used, for example, inorganic oxides such as indium tin oxide (ITO) and inorganic fluorides such as fluorine-doped tin oxide (FTO). These oxides and fluorides, however, are hard and brittle, and liable to crack or be exfoliated even with slight bending or deflection; thus, they are poor in machinability. In addition, indium is not abundant and will possibly be deficient in future, on a resource basis.
As other conductive material than the inorganic oxides and fluorides, there have been proposed conductive resin compositions in which conductive particulates such as those of metal or carbon are dispersed in a binder resin. Such conductive resin compositions, however, are poor in light-transmitting property and are not suited to uses where transparency is required.
On the other hand, carbon nanotubes, the existence of which was discovered in 1991, have been attracting attention because of their peculiar electrical characteristics of turning to be metallic or semiconducting depending on small changes in structure, as well as their mechanical characteristics of being flexible and tough. Therefore, basic researches and applied researches of carbon nanotubes have been made vigorously. In recent years, as one of the applied researches, there have been proposed transparent conductive films in which metallic carbon nanotube is used as a conductive material, paying attention to its extremely high electroconductive property, and methods for producing these films.
For example, Japanese Patent Laid-Open No. 2004-230690 (Claim 1, pp. 3-6, Table 1, hereinafter referred to as Patent Document 1) proposes an antistatic transparent resin plate in which an antistatic layer having carbon nanotubes contained in a transparent thermoplastic resin is formed on at least one side of a transparent thermoplastic resin substrate. It is described in Patent Document 1 that carbon nanotubes in a discrete (non-entangled) state in the antistatic layer or bundles each composing of a plurality of carbon nanotubes and being in a discrete (non-entangled) state are dispersed in the thermoplastic resin while making contact with one another. The antistatic layer is formed by coating the substrate with a coating liquid containing a powder of the thermoplastic resin, the carbon nanotubes, and a dispersant.
In the configuration of the antistatic layer and the production method as above-mentioned, however, mutual contact of the carbon nanotubes is obstructed by the dispersant, making it very difficult to utilize the excellent electroconductive property intrinsic of the carbon nanotubes. Therefore, in order to obtain a desired electroconductivity, it may be necessary to add a large amount of carbon nanotubes, which would cause a lowering in transparency. Besides, it is difficult to uniformly disperse the carbon nanotubes in the thermoplastic resin, so that it may be impossible to obtain a uniform electroconductive property in the antistatic layer.
Taking this into consideration, Japanese Patent Laid-Open No. 2006-519712 (Claims 1 and 2, pp. 6-11, FIGS. 2 and 4, hereinafter referred to as Patent Document 2) proposes a conductive molded body including a substrate, and a conductive layer which is formed on a surface of the substrate and which contains thin conductive fibers dispersed therein. In this conductive molded body, at least some portions of the conductive fibers are fixed (anchored) to the substrate, while at least other portions of the conductive fibers are protruding from the outermost surface of the conductive layer, and the conductive fibers are making electrical contact with each other at their portions protruding from the outermost surfaces or at their portions fixed to the substrate.
In Example 1 of Patent Document 2, an example of production of the above-mentioned conductive molded body is shown in which after the conductive layer is formed in the same manner as the antistatic layer in Patent Document 1, the conductive layer is subjected to hot pressing so that spring-back forces of the carbon nanotubes cause the carbon nanotubes to protrude from the surface of the thermoplastic resin layer. In this example, the surface resistivity of the conductive layer was lowered from 2.4×1011Ω/□ to 7.7×107Ω/□ through the hot pressing. Patent Document 2 describes that the reason for the lowering in surface resistivity is as follows. In the conductive layer yet to be hot pressed, the carbon nanotubes are buried in the thermoplastic resin layer. In the conductive layer having been hot pressed, on the other hand, portions of the carbon nanotubes are protruding from the thermoplastic resin layer. When the protruding portions of the carbon nanotubes touch each other in the outside of the thermoplastic resin layer, the electric insulator (thermoplastic resin) is not present between the mutually touching carbon nanotubes, so that the resistance between the carbon nanotubes is lowered.
Besides, in Example 2 of Patent Document 2, an example of production of the conductive molded body is shown in which a polyethylene terephthalate (PET) film is coated with a coating liquid prepared by mixing single-wall carbon nanotubes and a dispersant (a polyoxyethylene-polyoxypropylene copolymer) into a mixed solvent of isopropyl alcohol and water. After a conductive material composed only of the carbon nanotubes and the dispersant is thus formed, a urethane acrylate solution is applied thereto, whereby joints between the surface of the PET film and the carbon nanotubes making contact with the surface are covered with a urethane acrylate resin layer. In this example, the content of the carbon nanotubes in the coating liquid was 0.003 mass %, and the content of the dispersant was 0.05 mass %.
An application of the semiconductor characteristics of semiconducting carbon nanotubes is described, for example, in M. Lee et al., Nature nanotechnology, 2006, 1, 66 (hereinafter referred to as Non-Patent Document 1). In this example, a conductive layer composed only of single-wall carbon nanotubes is formed, and a transistor of a field effect transistor (FET) structure in which the conductive layer is used as a channel layer and a substance sensor utilizing the same are fabricated. Non-Patent Document 1 shows a method of forming the conductive layer wherein a substrate is immersed in a dispersion of the single-wall carbon nanotubes in 1,2-dichlorobenzene, and the single-wall carbon nanotubes are adhered to the substrate by utilizing the affinity between the single-wall carbon nanotubes and the substrate. Besides, the Non-Patent Document 1 describes that the single-wall carbon nanotubes show strong affinity for exposed surfaces of most of substrate materials, and that examples of such substrate materials include gold, silicon dioxide, glass, silicon, and aluminum.
In addition, Q. Cao et al., Advanced Materials, 2006, 18, 304 (hereinafter referred to as Non-Patent Document 2) shows an example of fabrication of a transparent and flexible thin film transistor (TFT). In this example, a method is adopted in which carbon nanotubes are formed on a silicon oxide layer on a surface of a silicon wafer by a chemical vapor deposition (CVD) technique, and the carbon nanotubes thus formed are transfer printed onto a photo-curing epoxy pressure sensitive adhesive layer formed on a PET film. By using this method, a gate electrode, a semiconductor layer, and source and drain electrodes are formed, whereby all the other members than a gate insulator layer of the TFT are formed by use of the carbon nanotubes.
Further, Y. Zhou et al., Applied Physics Letters, 2006, 88, 123109 (hereinafter referred to as Non-Patent Document 3) proposes a method for forming a conductive material in which carbon nanotubes are distributed uniformly. In this method, a dispersion of carbon nanotubes is filtrated through a filter, and the carbon nanotubes deposited uniformly on the filter are transferred onto a transparent substrate.